Line-1, 2nd button up, will light after 9+ dialing. once you establish a call you can put it on hold but pressing line-1 will not get the call back, it just beeps. You have to wait for the hold recall timer to expire to get the call back. And even then you have to pick-up the receiver, pressing the line button alone still does nothing.

If you dial #7 from an idle phone, you immediately get fast busy.